This invention relates to a method and an apparatus for determining characteristics of components of a communication channel, especially a transponder in a communication satellite under load.
The characteristics of a communication channel may change during the lifetime of the equipment used. Various tests may be performed not only at the beginning of but also repeatedly during the lifetime to verify that the communication channel meets predetermined specifications. Usually, these tests are performed without normal traffic, i.e. without the communication channel being used for transmission of a communication signal. This scenario will be explained in the following in greater detail with reference to communication satellites but without limiting the invention disclosed further below to only this application although the invention is specifically applicable in this field.
In a communication satellite, a communication channel is set up by a transponder of the satellite comprising several components like a receiving antenna, an input demultiplexer, a power amplifier, an output multiplexer, and a transmitting antenna. Transponder characteristics, such as amplitude response and group delay, are measured not only at the beginning of life of the spacecraft on the ground and, after launch, in orbit but also during the lifetime. These measurements are conventionally carried out without normal traffic on the transponder, i.e. without a payload signal being transmitted to and being re-transmitted by the transponder.
The necessity of switching off the payload signal during tests represents a considerable drawback not only for the user of the transponder, since communication is interrupted, but also for the operator of the satellite since the tests have to be performed in an expedited manner to keep the interruption as short as possible. In some cases it is impossible to interrupt communications via the communication channel so that the components of these channels cannot be tested after having enters into operation.
It is an object of the present invention to provide a method and an apparatus for determining characteristics of components of a communication channel, especially a transponder of a satellite, without the necessity to interrupt traffic via the communication channel.
This object and other objects are achieved by a method for determining characteristics of components of a communication channel which is designed for transmitting a payload signal at a predetermined level, comprising: generating a first pseudo noise signal PN(t); modulating a clean carrier signal f(t) with said first pseudo noise signal PN(t) to generate a PN modulated clean carrier signal s(t); transmitting said PN modulated clean carrier signal s(t) simultaneously with said payload signal through said communication channel at a level below the level of said payload signal; receiving a receive signal sxe2x80x2(t) corresponding to said PN modulated clean carrier signal s(t) after having traveled through said communication channel; correlating said receive signal sxe2x80x2(t) with said first pseudo noise signal PN(t) to generate a recovered carrier signal fxe2x80x2(t); and determining characteristics of components of the communication channel on the basis of a comparison of said clean carrier signal f(t) and said recovered carrier signal fxe2x80x2(t).
Advantageously, the level of said PN modulated clean carrier signal s(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said first pseudo noise signal PN(t) is a binary pseudo noise sequence, which is preferably generated by means of a feed back shift register.
A chiprate of said first pseudo noise signal PN(t) is less than 5 MChip/s and preferably less than or equal to 2.5 MChip/s.
In a further embodiment, said correlating of said receive signal sxe2x80x2(t) and said first pseudo noise signal PN(t) is achieved by delaying said first pseudo noise signal PN(t) and multiplying the delayed first pseudo noise signal PN(t) and said receive signal sxe2x80x2(t).
To generate a reference, the method according to the invention further comprises: generating a second pseudo noise signal PNR(t); modulating a reference carrier signal fR(t) with said second pseudo noise signal PNR(t) to generate a PN modulated reference carrier signal sR(t); transmitting said PN modulated reference carrier signal SR(t) through said communication channel at a level below the level of said payload signal; receiving a reference receive signal sRxe2x80x2(t) corresponding to said PN modulated reference carrier signal sR(t) after having traveled through said communication channel; correlating said reference receive signal sRxe2x80x2(t) with said second pseudo noise signal PNR(t) to generate a recovered reference carrier signal fRxe2x80x2(t); and determining characteristics of components of the communication channel also on the basis of a comparison of said reference carrier signal fR(t) and said recovered carrier signal fRxe2x80x2(t).
Advantageously, the level of said PN modulated reference carrier signal sR(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said second pseudo noise signal PN(t) is a binary pseudo noise sequence which is preferably generated by means of a feed back shift register.
In a further embodiment, said correlating of said reference receive signal sRxe2x80x2(t) and said second pseudo noise signal PNR(t) is achieved by delaying said second pseudo noise signal PNR(t) and multiplying the delayed second pseudo noise signal PNR(t) and said reference receive signal sR(t) can also be transmitte through a different transponder of the satellite.
The method of the invention as characterized above is especially applicable when said communication channel is a transponder of a communication satellite. Said PN modulated reference signal sR(t) may be transmitted through the same transponder of the satellite, but then said second pseudo noise signal PNR(t) must not correlated with said pseudo noise signal PN(t). Said PN modulated reference signal sR(t) can also be transmitted through a different transponder of the satellite.
The characteristics of said communication channel may be group delay and amplitude response.
The above objects and other objects are also achieved by an apparatus for determining characteristics of components of a communication channel which is designed for transmitting a payload signal at a predetermined level, comprising first pseudo noise signal generating means for generating a pseudo noise signal PN(t); first modulating means for modulating a clean carrier signal f(t) with said first pseudo noise signal PN(t) to generate a PN modulated clean carrier signal s(t); transmitting means for transmitting said PN modulated clean carrier signal s(t) through said communication channel at a level below the level of said payload signal; receiving means for receiving a receive signal sxe2x80x2(t) corresponding to said PN modulated clean carrier signal s(t) after having traveled through said communication channel; and first correlating means for correlating said receive signal sxe2x80x2(t) with said pseudo noise signal PN(t) to generate a recovered carrier signal fxe2x80x2(t).
Advantageously, the level of said PN modulated clean carrier signal s(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said first pseudo noise signal generating means is a feed back shift register.
A chiprate of said first pseudo noise signal PN(t) is less than 5 MChip/s preferably less than or equal to 2.5 MChip/s.
In a further embodiment, the above apparatus comprises first delaying means for delaying said first pseudo noise signal PN(t).
To obtain a reference, the above apparatus further comprises second pseudo noise generating means for generating a second pseudo noise signal PNR(t); second modulating means for modulating a reference carrier signal fR(t) with said second pseudo noise signal PNR(t) to generate a PN modulated reference carrier signal sR(t); transmitting means for transmitting said PN modulated reference carrier signal sR(t) through said communication channel at a level below the level of said payload signal; receiving means for receiving a reference receive signal sRxe2x80x2(t) corresponding to said PN modulated reference carrier signal sR(t) after having traveled through said communication channel; and second correlating means for correlating said reference receive signal sRxe2x80x2(t) with said second pseudo noise signal PNR(t) to generate a recovered reference carrier signal fRxe2x80x2(t).
Advantageously, the level of said PN modulated reference carrier signal s(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said second pseudo noise signal generating means is a feed back shift register.
In a further embodiment, the above apparatus further comprises second delaying means for delaying said second pseudo noise signal PNR(t).
In summary, for determining characteristics of components of a communication channel, for example of a transponder in a communication satellite, a clean carrier signal f(t) is modulated with a pseudo noise signal PN(t) and transmitted through the communication channel at a level below the level of said payload signal which is designed for transmitting a payload signal. The received signal sxe2x80x2(t) is correlated with the same pseudo noise signal PN(t) to obtain a recovered carrier signal fxe2x80x2(t). The clean carrier signal f(t) and the recovered carrier signal fxe2x80x2(t) are used together to determine the desired characteristics. Since the PN modulated clean carrier signal s(t) is transmitted at a low level, it is possible to perform measurements without switching off the payload signal.
The most important advantage of the method and the apparatus according to the invention is of course that the payload signal does not have to be switched off for performing the measurements. This limits considerably the downtime required for maintenance and verification of the communication channel, and thus increases availability of services.
Another very important advantage is the fact that with this method and apparatus, it is possible to measure characteristics of components of the communication channel under realistic conditions. For example, in a satellite transponder the IMUX and OMUX filters are waveguide filters and the characteristics of these filters are changing with the temperature. Normally, the filters are not uniformly heated during operation but are heated depending on the payload signal. When the payload signal is switched off the temperature distribution changes compared to normal operation even if the test signals provide a certain power for heating the filters. Thus, with conventional methods characteristics cannot be determined under conditions present in the communication channel under load. In addition, in the proposed method the spectral power density of the measurement signal is considerably lower than the spectral power density of the payload signal, so that it is possible to characterise the behaviour of the communications channel under the most realistic circumstances.
A further advantage of the invention is that in the case of a satellite communication channel the conversion frequency of the uplink/downlink can be measured without interruption of the payload signal and simultaneously with the other measurements.